WordNet records no synonym and no antonym for legionnaires' disease. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
acute (sometimes fatal) lobar pneumonia caused by bacteria of a kind first recognized after an outbreak of the disease at an American Legion convention in Philadelphia in 1976; characterized by fever and muscle and chest pain and headache and chills and a dry cough
Synonyms, antonyms and the broader and narrower relations for legionnaires' disease come from Princeton University’s WordNet. Full sources and licences.